The present invention relates to a resist composition suitable for lithography by the action of such high-energy rays as far ultraviolet rays (including excimer laser and others), electron beam, X-ray or other rays.
With an increasing demand for higher integration of integrated circuits in recent years, formation of submicron patterns has been required. Particularly, since lithography with the use of krypton fluoride (KrF) excimer laser or argon fluoride (ArF) excimer laser makes the production of integrated circuits of 64 M DRAM to 1 G DRAM possible, the lithographic process is drawing attention. As a resist suitable for the above-mentioned excimer laser lithographic process, so-called a chemical amplification type resist utilizing an acid catalyst and the chemical amplification effect has been increasingly adopted. In the chemical amplification type resist, an acid generated from an acid generating agent at areas irradiated with a ray is diffused in the following heat treatment (post exposure bake: hereinafter sometimes abbreviated as PEB) and a reaction catalyzed by this acid changes the solubility of the exposed areas in an alkaline developer, providing positive or negative patterns.
For a positive resist of the chemical amplification type, particularly a positive resist for KrF excimer laser lithography, a poly(hydroxystyrene) resin with part of its phenolic hydroxyl group protected by a group which undergoes cleavage by the action of an acid is employed, in many cases in combination with an acid generating agent. As the group which undergoes cleavage by the action of an acid, from the viewpoints of resolution, sensitivity, and others, those forming acetal-type bonds with oxygen atoms derived from phenolic hydroxyl groups, such as resins having a structure in which tetrahydro-2-pyranyl, tetrahydro-2-furyl, or 1-ethoxyethyl is bound to an oxygen atom, are arousing interest. However, even the use of such resins has been faced with limitations in improvement in resolution.
Moreover, in the formation of a pattern by photolithography, generally, variations in exposure dose are liable to lead to variations in the finished dimensions of the resulting resist pattern, indicating its small exposure latitude (also referred to as exposure margin). As described above, resist compositions conventionally known in the art have limits in resolution, sensitivity, exposure capacity, and others. Furthermore, since the manufacture of integrated circuits involves dry etching through a resist pattern formed by photolithography as a mask, the resist employed therefor is also required to have heat resistance and dry etching resistance.
The object of the present invention is to provide a chemical amplification type positive resist composition excellent in such properties as sensitivity, resolution, heat resistance, the ratio of residual thickness, coatability, exposure latitude, dry etching resistance, and others, particularly one that is further improved in resolution and exposure latitude.
The inventors of the present invention made intensive studies to achieve the above-mentioned object and have found that the use of a hydroxystyrene/3-hydroxy-1-adamanty methacrylate copolymer as a resin component for the chemical amplification type positive resist provides excellent performance. The present invention was accomplished based on this finding.
Accordingly, the present invention provides a chemical amplification type positive resist composition containing: a resin which has a hydroxystyrene-based polymerization unit, a 3-hydroxy-1-adamantyl methacrylate-based polymerization unit, and a polymerization unit having a group unstable toward an acid, and, though insoluble or hardly soluble in an alkali in itself, becomes alkali-soluble after the acid-unstable group described above has been cleaved by the action of an acid; and an acid generating agent.
Although the resin component which is a main constituent of the resist composition of the present invention is insoluble or hardly soluble in an alkali in itself, it is rendered alkali-soluble through a reaction by the action of an acid, and has a polymerization unit resulting from the opening of the double bond of the vinyl group of hydroxystyrene, and a polymerization unit which results from the opening of the double bond of the vinyl group of 3-hydroxy-1-adamantyl methacrylate, as essential constituents. These polymerization units can be represented by the following formulae (I) and (II), respectively. 
In the polymerization unit formed from hydroxystyrene shown by the formula (I), the position of the hydroxyl group is not particularly restricted, but the polymerization unit is generally one that is formed from p-hydroxystyrene.
Moreover, this resin is insoluble or hardly soluble in an alkali in itself. However, for becoming alkali-soluble through a chemical reaction by the action of an acid, it comprises a polymerization unit having a group unstable toward an acid. Usually, the group unstable toward an acid has been introduced to the resin so as to protect such an alkali-soluble group as hydroxyl group and carboxyl group. Concrete examples of such acid-unstable group for protecting an alkali-soluble group include tert-butyl, tert-butoxycarbonyl, an acetal-type group of the following formula (III): 
wherein, R1 represents for an alkyl group having 1 to 4 carbon atoms, R2 represents an alkyl group having 1 to 6 carbon atoms or a cycloalkyl group having 5 to 7 carbon atoms, or R1 and R2 together form a trimethylene or tetramethylene chain, and a 2-alkyl-2-adamantyl represented by the following formula (IV): 
wherein, R3 represents an alkyl group having 1 to 4 carbon atoms. These groups each substitutes for the hydrogen atom of the hydroxyl or carboxyl group.
Those suitable as the acetal-type group represented by the formula (III) include tetrahydro-2-furyl, tetrahydro-2-pyranyl, 1-ethoxyethyl, 1-isopropoxyethyl, 1-isobutoxyethyl, 1-ethoxypropyl, 1-ethoxy-2-methylpropyl, and 1-cyclohexyloxyethyl. Of these, 1-ethoxyethyl, 1-isobutoxyethyl, 1-isopropoxyethyl, 1-ethoxypropyl, and others are preferred. Moreover, examples of suitable 2-alkyl-2-adamantyl groups represented by the formula (IV) are 2-methyl-2-adamantyl and 2-ethyl-2-adamantyl.
Among the groups unstable toward an acid enumerated above, tert-butoxycarbonyl usually substitutes for hydroxyl group and a 2-alkyl-2-adamantyl group represented by the formula (IV) usually substitutes for carboxyl group. Other tert-butyl and acetal-type groups represented by the formula (III) can substitute for both hydroxyl group and carboxyl group.
In the case of a resist for KrF excimer laser exposure, the group unstable toward an acid is generally such as to protect the hydroxyl group of a hydroxystyrene unit. A unit in which the acid-unstable group is bound to the hydroxyl group of a hydroxylstyrene unit can be represented by the following formula (V): 
In the formula, Q represents the acid-unstable group. Tert-butyl, tert-butoxycarbonyl, acetal-type groups represented by the formula (III) shown above, and the like are mentioned as concrete examples of Q in this formula. Among these, the acetal-type groups represented by the above-shown formula (III) are preferred. Its preferred embodiment as a polymerization unit can be represented by the following formula 
wherein, R1 and R2 have the same meanings as defined in the formula (III). A particularly preferred is the case where R1 in the formula (III) or (VI) is an alkyl group having 1 to 4 carbon atoms and R2 is an alkyl group having 1 to 6 carbon atoms or a cycloalkyl having 5 to 7 carbon atoms.
The resin having a hydroxystyrene-based polymerization unit, a 3-hydroxy-1-adamantyl methacrylate-based polymerization unit, and such a polymerization unit having a group unstable toward an acid as was described above can be produced, for example, in the following manner. Firstly, in the case where tert-butyl serves as the acid-unstable group and substitutes for the hydroxyl group of hydroxystyrene, the resin can be produced by, after tert-butoxystyrene and 3-hydroxy-1-adamantyl methacrylate have been copolymerized in a conventional manner, hydrolyzing the resulting copolymer to an appropriate degree thereby to convert part of the tert-butoxy group to hydroxyl group. Moreover, when the group unstable toward an acid is tert-butoxycarbonyl and substitutes for the hydroxyl group of hydroxystyrene, the resin can be produced by allowing the copolymer of hydroxystyrene and 3-hydroxy-1-adamantyl methacrylate to react with di-tert-butyl dicarbonate.
Secondly, in the case where the group unstable toward an acid is an acetal-type group of the formula (III) and substitutes for the hydroxyl group of hydroxystyrene, the resin can be produced by allowing the copolymer of hydroxystyrene and 3-hydroxy-1-adamantyl methacrylate to react with an unsaturated ether compound represented by the following formula (IIIa): 
wherein, R2 has the same meaning as defined above; R4 and R5 each independently represents hydrogen or an alkyl group with the proviso that the number of carbon atoms in total is from 0 to 3 or R5 represents hydrogen; and R2 and R4 together form a trimethylene or tetramethylene chain.
On the other hand, in the case where the group which is unstable toward an acid is tert-butyl, an acetal type group represented by the formula (III), or a 2-alkyl-2-adamantyl group represented by the formula (IV) and substitutes for carboxyl group, usually, the resin can be produced by copolymerizing an unsaturated compound constituting a carboxylate with any of these groups, with hydroxystyrene and 3-hydroxy-1-adamantyl methacrylate. As the unsaturated compound constituting a carboxylate with a group unstable toward an acid, there are exemplified unsaturated aliphatic carboxylates such as acrylates and methacrylates and unsaturated alicyclic carboxylates such as norbornene carboxylate, tricyclodecene carboxylate, and tetracyclodecene carboxylate.
Copolymerization of hydroxystyrene or tert-butoxystyrene with 3-hydroxy-1-adamantyl methacrylate, and, if required, a monomer having a group unstable toward an acid and/or other monomers can be effected in accordance with a conventional manner. For example, a method comprising dissolving monomers as starting materials in an appropriate solvent, adding a polymerization initiator thereto to cause a polymerization reaction, and keeping the reaction proceeding while heating or removing heat therefrom can be adopted. As the reaction solvent, there can be employed an alcohol such as methanol, ethanol, 2-propanol, and tert-butanol, an aromatic hydrocarbon such as benzene, toluene, and xylene, or an ether such as tetrahydrofuran and 1,4-dioxane. Moreover, as the polymerization initiator, there can be used an azo compound such as 2,2xe2x80x2-azobis(isobutylonitrile) and dimethyl2,2xe2x80x2-azobis(2-methylpropionate), a peroxide such as benzoyl peroxide and tert-butyl peroxide, a redox catalyst such as hydrogen peroxide/ferrous salt and benzoyl peroxide/dimethylaniline, or a metal alkyl compound such as butyllithium and triethylaluminium.
In the case where tert-butoxystyrene is employed as a starting material monomer, the copolymer is dissolved in a hydrophilic solvent and heated under acidic conditions, whereby the tert-butoxy group contained therein is hydrolyzed to be converted to a hydroxyl group and the copolymer becomes a hydroxystyrene/3-hydroxy-1-adamantyl methacrylate copolymer. Moreover, when introducing a group which is unstable toward an acid to the hydroxystyrene/3-hydroxy-1-adamantyl methacrylate copolymer, following such a procedure as to accord to an ordinary protective group-introducing reaction will be sufficient. For example, in the case where an acetal-type group expressed by the formula (III) shown above is introduced, part of the hydroxyl group in the copolymer can be converted to an acetal-type group represented by the formula (III) by dissolving the copolymer as a starting material in a solvent and allowing it to react with an unsaturated ether compound represented by the formula (IIIa) in the presence of an acidic catalyst. As the acidic catalyst employed in this reaction, inorganic acids such as hydrochloric acid and sulfuric acid and organic acids such as p-toluenesulfonic acid and camphorsulfonic acid are available.
As to the resin component which constitutes the positive resin composition of the present invention, hydroxystyrene-based polymerization unit, 3-hydroxy-1-adamantyl methacrylate-based polymerization unit, and polymerization unit having a group unstable toward an acid as were described above are essential. Further, it may contain other polymerization units, such as styrene, acrylonitrile, methyl methacrylate, and methyl acrylate. Further, for improving the transparency, it may be partially hydrogenated, or an alkyl group, an alkoxy group, or the like may be introduced to the phenol ring with the proviso the resin composition remains alkali-soluble. However, that the hydroxystyrene-based polymerization units and 3-hydroxy-1-adamantyl methacrylate polymerization units account for, in total, 50 mol % or more of the all polymerization units of the resin will be advantageous. The ratio of these polymerization units is, in terms of the molar ratio of hydroxystyrene unit: 3-hydroxy-1-adamantyl methacrylate unit, usually in the range of from 99:1 to 80:20, preferably in the range of from 95:5 to 85:15. Moreover, the polymerization units having a group unstable toward to an acid usually account for 50 mol % or less of all the polymerization units of the resin, preferably not less than 10 mol % nor more than 45 mol %.
Incidentally, in the case where the hydroxystyrene/3-hydroxy-1-adamantyl methacrylate copolymer is reacted with a compound which is for introducing an acid-unstable group, such as di-tert-butyl dicarbonate or an unsaturated ether compound represented by the above-shown formula (IIIa), a protective group may be introduced not only to the hydroxyl group of the hydroxystyrene unit but also to the hydroxyl group of the 3-hydroxy-1-admantyl methacrylate unit. In the case of the presence of such a unit in which the hydroxyl group of 3-hydroxy-1-adamantyl methacrylate is protected by a group unstable toward an acid, considering it to be categorized as a polymerization unit having an unstable group is rational.
Next, the acid generating agent which is the other constituent of the positive resist composition generates an acid upon decomposition caused by allowing a ray exemplified by light or electron beam to act on the substance itself or a resist composition containing this substance. The acid generated from the acid generating agent acts on the above-described resin, resulting in the cleavage of the acid-unstable group existing in this resin. Examples of such acid generating agent include onium salt compounds, s-triazine type organic halogen compounds, sulfonic compounds, and sulfonates. To be concrete, the following compounds are exemplified as the acid generating agent.
Diphenyliodonium trifluoromethanesulfonate, 4-methoxyphenylphenyliodonium hexafluoroantimonate, 4-methoxyphenylphenyliodonium trifluoromethanesulfonate, bis(4-tert-butylphenyl)iodonium tetrafluoroborate, bis(4-tert-butylphenyl)iodonium hexafluorophosphate, bis(4-tert-butylphenyl)iodonium hexafluoroantimonate, bis(4-tert-butylphenyl)iodonium trifluromethanesulfonate, triphenylsulfonium hexafluorophosphate, triphenylsulfonium hexafluoroantimonate, triphenylsulfonium trifluromethanesulfonate, 4-methylphenyldiphenylsulfonium perfluorobutanesulfonate, 4-methylphenyldiphenylsulfonium perfluorooctanesulfonate, 4-methoxyphenyldiphenylsulfonium hexafluoroantimonate, 4-methoxyphenyldiphenylsulfonium trifluoromethanesulfonate, p-tolyldiphenylsulfonium trifluoromethanesulfonate, 2,4,6-trimethylphenyldiphenylsulfonium trifluoromethanesulfonate, 4-tert-butylphenyldiphenylsulfonium trifluoromethanesulfonate, 4-phenylthiophenyldiphenylsulfonium hexafluorophosphate, 4-phenylthiophenyldiphenylsulfonium hexafluoroantimonate, 1-(2-naphthoylmethyl)thiolanium hexafluoroantimonate, 1-(2-naphthoylmethyl)thiolanium trifluoromethanesulfonate, 4-hydroxy-1-naphthyldimethylsulfonium hexafluoroantimonate, 4-hydroxy-1-naphthyldimethylsulfonium trifluoromethanesulfonate, 2-methyl-4,6-bis(trichloromethyl)-1,3,5-triazine, 2,4,6-tris(trichloromethyl)-1,3,5-triazine, 2-phenyl-4,6-bis(trichloromethyl)-1,3,5-triazine, 2-(4-chlorophenyl)-4,6-bis(trichloromethyl)-1,3,5-triazine, 2-(4-methoxyphenyl)-4,6-bis(trichloromethyl)-1,3,5-triazine, 2-(4-methoxy-1-naphthyl)-4,6-bis(trichloromethyl)-1,3,5-triazine, 2-(benzo[d][1,3]dioxolan-5-yl)-4,6-bis(trichloromethyl)-1,3,5-triazine, 2-(4-methoxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine, 2-(3,4,5-trimethoxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine, 2-(3,4-dimethoxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine, 2-(2,4-dimethoxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine, 2-(2-methoxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine, 2-(4-butoxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine, 2-(4-pentyloxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine, 1-benzoyl-1-phenylmethyl p-toluenesulfonate (what is called benzointosylate), 2-benzoyl-2-hydroxy-2-phenylethyl p-toluenesulfonate (what is called xcex1-methylolbenzointosylate), 1,2,3-benzenetoluyl trismethanesulfonate, 2,6-dinitrobenzyl p-toluenesulfonate, 2-nitrobenzyl p-toluenesulfonate, 4-nitrobenzyl p-toluenesulfonate, diphenyl disulfone, di-p-tolyl disulfone, bis(phenylsulfonyl)diazomethane, bis(4-chlorophenylsulfonyl)diazomethane, bis(p-tolylsulfonyl)diazomethane, bis(4-tert-butylphenylsulfonyl)diazomethane, bis(2,4-xylylsulfonyl)diazomethane, bis(cyclohexylsulfonyl)diazomethane, (benzoyl) (phenylsulfonyl)diazomethane, N-(phenylsulfonyloxy)succinimide, N-(trifluoromethylsulfonyloxy)succinimide, N-(trifluoromethylsulfonyloxy)phthalimide, N-(trifluoromethylsulfonyloxy)-5-norbornene-2,3-dicarboxy imide, N-(trifluoromethylsulfonyloxy)naphthalimide, and N-(10-camphorsulfonyloxy)naphthalimide.
Moreover, generally in the chemical amplification type positive resist, it has been known that the addition of an organic basic compound as a quencher improves the degree of deterioration in performance due to the deactivation resulting from standing after exposure to light. It is preferred that, also in the present invention, an organic basic compound, particularly a nitrogen-containing basic organic compound is blended thereinto as a quencher. Concrete examples of such nitrogen-containing basic organic compound include amines represented by the following respective formulae. 
wherein, R11, R12, R13, R14, and R15 each independently represents hydrogen, or an alkyl, cycloalkyl, aryl or alkoxy group which may be substituted by a hydroxyl group, and A represents an alkylene, carbonyl, or imino group. In the formulae, the alkyl and alkoxy groups represented by R11 to R15 each may have about 1 to 6 carbon atoms, and the cycloalkyl group may have about 5 to 10 carbon atoms, and the aryl group may have about 6 to 10 carbon atoms. Moreover, the alkylene group denoted by A may have about 1 to 6 carbon atoms and may be a straight or branched chain. Furthermore, the hindered amine compound having a hindered piperidine skeleton, disclosed in Japanese Patent Application No. 9-208864 which is the application previously filed by the applicants of the present invention, also can serve as the quencher.
The resist composition of the present invention is preferred to contain, based on the total solid content, 80 to 99.8% by weight of the resin component and 0.1 to 20% by weight of the acid generating agent. In the case where a nitrogen-containing basic organic compound is incorporated therein as a quencher, it is preferably used in an amount of, based on the total solid content in the resin composition, 10% by weight or less. This composition may contain, if necessary, a variety of additives such as sensitizers, dissolution inhibitors, other resins, surfactants, stabilizers, and dyes in small amounts.
This resist composition is usually in the form of a resist solution in which the above-mentioned components are dissolved in a solvent, and coated on a substrate such as a silicone wafer by a conventional manner such as spin coating. The solvent to be employed need only be capable of dissolving each component, have an appropriate drying rate, and be able to provide a uniform and smooth coat after it has been evaporated off. Such a solvent as is commonly employed in the art can be used. For example, there are mentioned glycol ether esters such as ethyl cellosolve acetate, methyl cellosolve acetate, and propylene glycol monomethyl ether acetate, esters such as ethyl lactate, butyl acetate, amyl acetate, and ethyl pyruvate, ketones such as acetone, methyl isobutyl ketone, 2-heptanone, and cyclohexanone, cyclic esters such as xcex3-butyrolactone, and alcohols such as 3-methoxy-1-butanol. These solvents can be used either singly or as a combination of two or more.
The resist film coated and dried on the substrate is exposed to light for patterning. Thereafter, after heat treatment (PEB) for accelerating a protective group-eliminating reaction has been conducted, it is developed with an alkaline developer. Although the alkaline developer employed here can be any of a variety of alkaline aqueous solutions that are used in the art, generally, an aqueous solution of tetramethyl ammonium hydroxide or (2-hydroxyethyl)trimethyl ammonium hydroxide (so-called choline) is in many cases employed.